Synchronizing apparatus for television receivers



March 1934- F. VON OKOLICSANYI ET AL 1,950,331

SYNCHRQNIZING APPARATUS FOR TELEVISION RECEIVERS Filed Feb. 12, 1930 Patented Mar. 13, 1934 UNITED STATES PATENT OFFICE SYNCHRONIZING APPARATUS FOR TELE- VISION RECEIVERS Application February 12, 1930, Serial No. 427,854 In Germany December 7, 1929 1 Claim.

Two methods are mainly known for the synchronization of television receiving apparatus which have hi.herto given more or less satisfactory results. The one method consists in this that without using any synchronizing connection between the transmitter and receiver, in order to obtain constant running, a local frequency bea' or is arranged in the transmitter and in the receiver, both of which have the same frequency of oscillation.

The second method consists in synchronizing the receiver by the frequency of the line changes of the received photoelectric curren's, however, no reliable solution has hitherto been obtained in this way.

Both methods are reliable as long as the economical points of view are not taken into consideration, as is for instance the case in laboratory experimenis. The local frequency beater can produce a perfect constant running for the de sired period of time. However, for practical purposes only a frequency beater which is comparatively cheaper and more simple comes into question in which case, owing to the smaller precision, aconstant running lasting only a few minutes .is considered to be a good result.

The synchronization by means of aliernating currents having the frequency of the line changes, is reliable so long as the picture to be transmitted is stationary. However, when the picture is moved in the transmitter, thus, for insl ance, in the case of transmissions of movable objects at a distance, or of kinematographic films, the maxima of the frequencies of the line changes are displaced. This causes an irregularity in the running of the driving synchronous motor of the receiver, so that the received picture is rocked about. By taking certain measures, for instance by using a pronounced bright or dark picture edge, this rocking can be prevented to a certain extent. It cannot, however, be entirely removed.

In order to avoid this drawback, it has already been proposed to effect the synchronization by means of a local frequency beater, such as for instance a tuning fork interrupter or the like, which is controlled by the receiving alternating currents of line frequency and the natural frequency of which is equal to the said line frequency.

This known arrangement could not, however, completely avoid the existing drawbacks since, in

the case of the above mentioned displacement of the line frequency from the normal value, which may occur with moving pictures and when the details are very pronounced, such as bright or dark places in the picture, the local frequency heaters are too strongly damped. Moreover, in the case of local frequency bea'ers as hitherto used, for instance tuning fork interrupters, the frequency is not sufficiently constant, so that the picture received in the picture window jumps easily. Further, the natural frequency varies considerably wi h the temperature. In the case of other frequency heaters, for instance piezo crystals, a regulation of the frequency, such as required when passing over from one transmission to another kind of transmission with a different number of picture lines is entirely impossible. The drawbacks above referred to are avoided according to the present invention by use being made of a local frequency bea er consisting of a thermionic tube generator whichis connected in such a manner as in the case of transmitting modulating connections, for instance grid modulation.

In this way the advantage is attained that the synchronization remains entirely independent of the kind of picture to be transmitted and of the picture changes and is cons antly maintained independent of the presence of a picture. Owing to the positive control of the constant running from the transmitter, this constant running can be ensured for any desired period of lime which has not been possible with the local frequency beaters as hitherto used.

The resonance curve of the thermionic tube generator must not be too pronounced but also not too fiat. In this way it is possible to let the thermionic tube generator oscillate exactly with the frequency of the line changes by controlling it through the same also when its natural frequency does not exactly correspond to the said frequency of the line changes. Thus it is possible by means of simple and cheap thermionic tube generators, to effect a sufliciently exact adjustment in order to obtain the desired exact tempo after the switching on of the photoelectric current. Experiments have also shown that even in the case of a comparatively rough adjustment of the local frequency beater, no beat oscillations are produced and that the local frequency beater follows the tempo of the frequency of the line changes also when the amplitude of the superposed photoelectric current is small as compared with the amplitude of oscillation of the local frequency beater.

The requirement above referred to that the resonance curve shall not be too pronounced and not too flat may for instance be attained, by means of an oscillatory circuit the coil of which has no or only a small iron core.

Naturally a too great flattening of the resonance curve would lead to this, that any influence in the character of the picture may disturb the exact constancy of the alternating frequency which is delivered by the thermionic tube generator to the receiving synchronous motor.

Instead of making the natural frequency of the thermionic tube generator equal to the frequency of the line changes, the one may be a harmonic of the other of the said frequencies.

The invention is illustrated by way of example and diagrammatically in the accompanying drawing.

Referring to the drawing, 1 is the tube of a thermionic tube generator with the known connections for the generation of oscillations. The oscillatory circuit 2 lies in the anode circuit of this tube and is tuned exactly or approximately to the frequency of the line changes of the received photoelectric currents. According to the standardization by the German Post Oflice, use is made for the picture assembling device of a Nipkow disc with 30 holes, rotatng at a speed of 12.5 revolutions per second; in this case the frequency of the line changes is 3'75. The tube is controlled by the photoelectric currents which are supplied by the receiver 3 over a transformer 4.

The secondary of the transformer is bridged over by an adjustable resistance 5, which renders possible a choking of the photoelectric currents controlling the tube 1. This choking must be effected to such an extent that the alternating currents controlling the tube just determine the frequency of the tube transmitter. If they were so large that they influenced also the amplitude of the tube 1, that is to say modulated the tube, the fault of the known methods of synchronizing by means of photoelectric currents would appear again and the picture in the receiver would rock when the picture character is changed in the transmitter.

The alternating currents obtained by the thermionic tube generator are amplified by the amplifier 6 and they serve for driving the receiving synchronous motor, in the case illustrated the phonic wheel '7, which in its turn drives the picture assembling device, in the case shown, the

' Nipkow disc 8. Behind the Nipkow disc there is arranged the receiving light relay 9, which is fed from a source of continuous current, the relay being controlled in a known manner by the photoelectric currents.

What we claim is:

A synchronizing apparatus for television receivers, comprising. the combination of a circuit for receiving photo-electric currents with a light relay sensitive to said photo-electric currents and connected to said circuit, a picture assembling device associated with said light relay, a synchronous motor for driving the picture assembling device, a local frequency beater consisting of a thermionic tube generator having a circuit tuned to the picture line scanning frequency, means comprising a receiver with a transformer adapted to supply alternating currents of picture line scanning frequency from the circuit receiving the photo-electric current to the thermionic tube generator, an adjustable resistance bridged across the secondary of said transformer and adapted to control the amplitude of said alternating current, whereby the frequency of the beater is determined without modulating the beater; and means for supplying alternating currents of picture line scanning frequency from said tuned circuit to the synchronous motor. FRANZ VON OKOLICSANYI. GUSTAV USER. 

